1. Field of the Invention
The present invention relates to time division multiple access (TDMA) communication systems, such as Global System for Mobile (GSM) transceivers, and in particular, to GSM transceivers capable of receiving General Package Radio Service (GPRS) data.
2. Description of the Related Art
As is well known, GSM is a second-generation cellular communication system standard that was developed to solve problems associated with the original analog cellular communication systems in Europe. GSM uses digital communication techniques, including frequency division duplexing (FDD) and a combination of time division multiple access (TDMA) and frequency division multiple access (FDMA) techniques to allow simultaneous access by base stations to multiple users.
Referring to FIG. 1, each communication channel in a GSM system is accessed on a time shared basis and is divided into time frames, which are 4.615 milliseconds long. Each time frame is divided into eight time slots 0-7, each of which is approximately 577 microseconds long. During normal used, i.e., during voice transmission and reception, the digitized voice information is packetized and transmitted during different time slots. For example, a receive operation for accepting incoming voice data may take place during time slot 0, while a transmit operation for sending voice data may take place during time slot 3. Additionally, a monitor function may be performed, e.g., during time slot 6, during which the signal strength for signals arriving from different base stations is monitored for power level. It is necessary to separate these active time slots 0, 3, 6 by two or more unused time slots so as to allow the common, or shared, phase lock loop (PLL) within the GSM handset to be set up and locked at the correct frequency for the receive, transmit or monitoring function. Minimum set up and lock times are required due to the use of integer PLL circuits. These integer PLL circuits are generally preferred over fractional PLL circuits since they are less complex and hence require significantly less integrated circuit area and less DC power.
Referring to FIG. 2, the transmit and receive functions of a GSM handset can be implemented using conventional transmitter and receiver circuit architectures. In this example, such functions can be implemented in an integrated form using a circuit 100 such as the LMX3411 manufactured by National Semiconductor Corporation of Santa Clara, Calif. As discussed in more detail below, this transmitter uses a modulation synthesizer architecture to convert an I/Q baseband representation to the desired transmit frequency. The receiver is a dual conversion receiver with channel selectivity provided by a SAW filter at the intermediate frequency (IF). In this particular integrated circuit 100, most of the transmitter and receiver functions are integrated within the chip.
Incoming signals received by the antenna 102 are forwarded to the low noise amplifier (LNA) stage 104, the bias for which is controlled by a control circuit within the chip 100. A band pass filter stage 106 filters the amplified signal prior to its frequency down conversion in the RF mixer stage 108 which is driven by a local oscillator signal provided by the RF PLL circuit 110 which uses an integer PLL 112. The resulting IF signal is filtered by an external SAW band pass filter 112 which provides the channel selectivity. The filtered IF signal is then amplified by an IF amplifier 114 having a programmable signal gain.
The amplified IF signal is converted to a quadrature representation in an I/Q mixer stage 116. The local oscillator signal is provided by the IF PLL circuit 114 which also uses an integer PLL 116. The resulting I and Q signals are low pass filtered and further amplified in a digitally programmable gain amplifier stage 118.
The transmitter architecture is based on a closed loop modulation of wide band frequency synthesizer. A wide band PLL 122 modulates an external voltage control oscillator (VCO) 126 having a loop filter bandwidth, as determined by the loop filter 124, sufficiently wide to correctly track the modulation inserted into the loop. A low pass filter 128 is used to filter out harmonics of the output signal prior to its amplification by the power amplifier 130 and transmission via the antenna 102. The output signal is also fed back to the transmitter down conversion mixer 118, which also receives its local oscillator signal from the RF PLL circuit 110. The resulting IF signal is band pass filtered and converted to a quadrature representation in the transmit quadrature mixer stage 120 in accordance with the outgoing signal data intended for transmission. These I and Q signals are low pass filtered and provided to the wide band PLL 122, along with the shared signal from the IF PLL circuit 114.
As discussed in more detail below, the aforementioned control and programming functionality is achieved by a way of an interface and control stage 120 which provides the programmable control data for the various control stages and programmable circuits.
Referring to FIG. 3, the interface and control stage 120 includes a set 200 of seven registers 201-207, which are used to store strings of control data ranging from 8 to 24 bits in length. The first register 201 controls power to the various stages of the circuit 100. The second register 202 provides the gain control information for the various programmable gain stages. The third 203 and fourth 204 registers store the control data for the N-counters within the RF 112 and IF 116 PLL stages (FIG. 2). The fifth register 205 stores the control data for the R-counters within the PLL stages 112, 116. The sixth register 206 stores the control data for the wide band PLL stage 122. The last register 207 stores miscellaneous control data used for various other functions within the circuit 100.
While the above-discussed transceiver architecture and control works well for most GSM services, particularly voice transmission and reception, the introduction of data services, such as GPRS, presents a problem concerning the lock time requirement of the PLL circuits. For example, for Class 12 GPRS, the worst case PLL lock time requirement is approximately 200 microseconds when using the above-discussed conventional approach for programming the receiver. In order to achieve this 200 microsecond lock time, the PLL circuits would need to be fractional PLL circuits which require significantly more integrated circuit area and DC power than integer PLL circuits such as those presently used in most cases. This means, that for normal voice operation where this faster lock time is not required, the power consumption of the handset is nonetheless increased significantly.
More specifically, a Class 12 GPRS operation will use up to five of the available eight time slots in a GSM time frame for data communication. For example, four receive slots and one transmit slot may be used, resulting in the following analysis (it will be understood that other permutations of receive and transmit slots may also be used and will yield the same analysis). As is known, a GSM time frame has eight time slots, each of which is approximately 577 microseconds. In this example, four receive operations, one transmit operation and one power monitor operation must be completed within the one time frame. Additionally, the transceiver needs to be set up for the subsequent time frame. Technically, there are two time slots allowed between the end of the receive operation and the beginning of the transmit operation within the time frame. However, since a GSM system is synchronized to the base station, the transmit signal may need to be transmitted prior to the actual start of the time slots within the intended handset so as to ensure that the signal arrives at the base station as the correct time. This is called xe2x80x9ctiming advance.xe2x80x9d The maximum specified timing advance is approximately 232 microseconds, or 63 bits, so the time interval between when the receive operation is completed and the transmit operation is begun may be reduced by up to 232 microseconds.
Additionally, when the handset needs to monitor different base stations for power level (often referred to as a received signal strength indicator, or RSSI, function), this is normally done between the receive and transmit operations (provided that there are at least two time slots between the receive and transmit time slots). The typical time duration of the monitor operation is approximately 232 microseconds, or the time equivalent of 64 bits. Accordingly, in order to achieve this, the transceiver must be programmed twice and the RF synthesizers must be locked twice within the remaining time.
The timing analysis is as follows. Two time slots total approximately 1,154 microseconds. Subtracted from this total time are the maximum timing advance of 232 microseconds and the duration of the monitor operation of 236microseconds. Further subtracted from this total time are two 8-bit write operations (for programming the power control registers in order to turn the receive and transmit sections on and off) totaling 16 microseconds (assuming a net programming speed of 1 bit per microsecond to include microprocessor calculation and setup). This leaves a net available time of 666 microseconds for setting up and locking the RF PLL stage two times, or 333 microseconds per PLL programming operation. With a net programming speed of 1 bit per microsecond and approximately 100 bits required to be written for each operation, this leaves a maximum time of 233 microseconds available for achieving phase lock. For the conventional integer PLL circuits used, this is not enough time for achieving phase lock.
Indeed, this time may be even less where a separate monitor, or RSSI, section must be cycled. In other words, there may be four 8-bit write operations required: (1) power down the receive section; (2) power up the monitor section; (3) power down the monitor section; and (4) power up the transmit section.
Accordingly, it would be desirable to have an improved technique for accommodating Class 12 GPRS operations within a GSM handset without requiring the increased cost and power requirements associated with the need for using fractional PLL circuits to met the faster phase lock time requirements.
A GSM receiver with time division duplexed operations for receiving a data signal, monitoring received signal strength and transmitting a data signal during a single time frame in accordance with one embodiment of the present invention uses a PLL control stage having multiple registers for storing control data that is preprogrammed prior to initiation of the receive, monitoring and transmit operations. These registers are preprogrammed with the control data during a time slot (e.g., time slot 7) within the prior time frame. This allows the programming for setting up the RF PLL to be reduced to the 16 bits (8 bits for power down plus 8 bits for power up) for the power control registers, thereby leaving more than 300 microseconds available for achieving phase lock and, in turn, thereby allowing standard integer PLL circuits to be used within the transceiver.
An apparatus including a GSM transceiver with time division duplexed operations for receiving a data signal and transmitting a data signal during a single time frame in accordance with one embodiment of the present invention includes a receiver stage, a transmitter stage, a signal generator stage and a controller stage. The receiver stage is configured to receive a local signal and in accordance therewith receive and frequency convert an incoming data signal during a first portion of a plurality of time slots within a present time frame and in accordance therewith provide a frequency converted incoming data signal. The transmitter stage is configured to receive the local signal and in accordance therewith receive and frequency convert an outgoing data signal during a second portion of the plurality of time slots within the present time frame and in accordance therewith provide a frequency converted outgoing data signal. The signal generator stage, coupled to the receiver and transmitter stages, is configured to receive first and second sets of control data and in accordance therewith provide the local signal at first and second signal frequencies during the first and second portions, respectively, of the plurality of time slots within the present time frame. The controller stage, coupled to the signal generator stage, is configured to receive and store the first and second sets of control data during a prior time frame and to provide to the signal generator stage one of the first and second sets of control data during a time interval between the first and second portions of the plurality of time slots within the present time frame.
A method for time division duplexing operations within a GSM transceiver for receiving a data signal and transmitting a data signal during a single time frame in accordance with another embodiment of the present invention includes the steps of:
storing first and second sets of frequency control data during a prior time frame;
reading the first set of frequency control data during a first time interval and in accordance therewith generating a local signal at a first frequency;
receiving and frequency converting an incoming data signal in accordance with the local signal at the first frequency during a first portion of a plurality of time slots within a present time frame and in accordance therewith generating a frequency converted incoming data signal;
reading the second set of frequency control data during a second time interval and in accordance therewith generating the local signal at a second frequency; and
receiving and frequency converting an outgoing data signal in accordance with the local signal at the second frequency during a second portion of the plurality of time slots within the present time frame and in accordance therewith generating a frequency converted outgoing data signal;
wherein one of the first and second time intervals is between the first and second portions of the plurality of time slots within the present time frame.